1. Field of the Invention
This invention relates generally to materials characterization. It relates in particular to the reproducible and uniform gripping of a film sample to be tested on the Rheovibron and Autovibron Viscoelastometer.
2. Description of Related Art
The Rheovibron Viscoelastometer (Toyo Measuring Instruments) has been used for many years for measuring the dynamic mechanical properties of polymers. See M. Takayanagi, "Viscoelastic Properties of Crystalline Polymers," Mem. Fac. Eng. Kyushu Univ., 23, 1 (1963). This instrument measures the temperature dependence of the complex modulus (E*) and loss tangent (tan .delta.) of viscoelastic materials at specific frequencies. A small amplitude sinusoidal tensile strain is imposed on one end of a clamped file sample and the sinusoidal tensile stress is measured at the other end. When operated manually the instrument is very labor intensive, requiring the constant attention of the operator for hours at a time if a wide temperature range is covered. In addition, sample mounting, maintaining sample tension, and maintaining sufficiently small intervals between data points are all operator dependent.
The Autovibron DDV-II-C (automated Rheovibron) was introduced by IMASS, Inc., in an effort to overcome many of these problems. See IMASS, Inc., Product Bulletin, Accord, Mass. (1980). This system provides increased sensitivity of measurement, constant tension on the film sample, automatic data logging, and short temperature intervals between successive measurements. These improvements are accomplished with the use of a multiprogrammer, programmable calculator and lock-in analyzer. In spite of the many advantages offered by the automated system, a problem still exists in achieving reproducible film clamping. The sample clamps or chucks provided with both the manual and automated systems for tensile measurements are "alligator clamps" illustrated in FIG. 1. The spring 11 on these clamps is depressed, opening the jaws 12 for film mounting, then released to allow the clamps to grip the film 13. The spring is fatigued over time with continued temperature cycling, making it difficult to reproducibly grip the samples. In addition, the gripping jaws of some of the clamps are not uniformly parallel, causing a pinching of the film in spots and little contact in others. Finally, perfecting the film alignment is limited by the necessity of positioning the film in clamps that are already in the instrument, thereby limiting operator visibility and maneuverability while causing unnecessary agitation to the transducer. Data obtained on the Rheovibron DDV-II-C (manual or automated) can show large variations between runs (on the order of 30 to 50%), due in large part to sample misalignment and yielding or slippage of the sample in the clamps. See D. J. Massa, J. R. Flick and S. E. B. Petrie, ACS Coat. and Plast. Prep., 35, 371 (1975), as well as A. R. Wedgewood and J. C. Seferis, Polymer, 22, 966 (1981).
In an effort to alleviate these problems, various new clamps were designed by prior art for testing polymers on the Rheovibron in tension in addition to clamps designed for shear and compression modes. See D. J. Massa, et al., supra, as well as T. Murayama, J. Appl. Polym. Sci., 20, 2593 (1976).
Although these new tensile clamps offer some improvements in clamping reproducibility over the alligator clamps, they require horizontal rather than vertical mounting of the films, and still remain operator dependent to assure proper film alignment.
The Massa, et al design 14 (see D. J. Massa, et al., supra) improves the clamp design by eliminating the spring clamping, replacing it with an Allen screw 15 and an alignment pin 16 to position the top portion of the clamp and hold the film (see FIG. 2). This design, however, still requires positioning the film and securing it while the clamps are mounted in the instrument. The side view of this horizontally clamped film is obstructed by the Rheovibron heating block, preventing an easy check of uniform gripping. Film alignment and uniform gripping in the holder are not assured, as the film can easily be aligned slightly off the clamp axis, whether through operator error or because of a slight torque imposed on the film in tightening the Allen screw.
The Wedgewood, et al clamps 17 (see S. R. Wedgewood, et al, supra) provide a definite advantage over the previous clamps in being able to mount and align a film sample 13 away from the instrument (see FIG. 3). As with the Massa design (FIG. 2) however, this design requires horizontal mounting, with the side (edge on) view of the film obstructed by the heating block. Holding the small clamps stationary while mounting the film is also awkward. More importantly, however, the circular design still allows for alignment of the clamp slightly off axes in the clamp holder.